I had the same problem.
I used some javascripts (jquery) to solve it.
my code in load action :
string script = @"
try{
$('#"+ lookupcontrol.ClientID +@"').find('a:first').click(function(event)
{
$('#" + lookupcontrol.ClientID + @"_lookup_operators').val('sw');
});
}
catch(e){
}";
ScriptManager.RegisterClientScriptBlock(this.Page,this.Page.GetType(),"Lookup SW", script, true);
Hope this help
Libert Damien